1. Field of the Invention
The present invention relates to a radio communication system in which the same information is transmitted from a radio station to a plurality of mobile stations, and a radio station, a mobile station, and down link transmission power control method that are suitable for use in the radio communication system.
2. Description of the Related Art
Conventionally, as a technique for simultaneously transmitting the same information from a radio station such as a base station or a radio station to a plurality of mobile stations, there are techniques such as the broadcast communication and the multicast communication.
As shown in FIG. 1, the broadcast communication is a technique for simultaneously transmitting the same information from one or a plurality of base stations (11 to 17) to all mobile stations (101 to 112) located in areas of them (see, for example, 23.041 Technical realization of Cell Broadcast Service (CBS) written by 3rd Generation Partnership Project Technical Specification Group Terminals, August 2000). As shown in FIG. 2, the multicast communication is a technique for simultaneously transmitting the same information from one or a plurality of base stations (11 to 17) to mobile stations (101, 103, 105, 106, 108, 109 and 110) of a specific group located in areas of them (see, for example, 25.324 Broadcast/Multicast Control BMC, December 2000, written by 3rd Generation Partnership Project Technical Specification Group Radio Access Network, December 2000).
Furthermore, as a technique whereby the radio station controls a predetermined down link transmission power in the conventional radio communication system when the same information is transmitted from the radio station to a plurality of mobile stations with the predetermined down link transmission power, there is a technique whereby the radio station controls a predetermined down link transmission power based on reception quality (such as received power, a ratio (SIR) of the same information of the received power to interference power, and an error bit rate of the same information) calculated by the mobile stations (see, for example, Japanese Patent Application Laid-Open Publication No. 2001-292096).
In the case where a radio station such as a base station or a radio network controller controls a predetermined down link transmission power in the conventional radio communication system, the radio station requests all subordinate mobile stations to transmit control information (such as the reception quality). Even if it is not necessary to control the predetermined down link transmission power because a mobile station is located near the radio station, the mobile station transmits the control information to the radio station. Therefore, there is a problem that control information that does not contribute to control of the predetermined down link transmission power increases and the traffic of up link radio resources increases.